Auditory brainstem response

In 1967, Sohmer and Feinmesser were the first to publish human ABRs recorded with surface electrodes, showing that cochlear potentials could be obtained non-invasively. In 1971, Jewett and Williston gave a clear description of the human ABR and correctly interpreted the later waves as arriving from the brainstem. In 1977, Selters and Brackman reported on prolonged inter-peak latencies in tumor cases (greater than 1 cm). In 1974, Hecox and Galambos showed that ABR could be used for threshold estimation in adults and infants. In 1975, Starr and Achor were the first to report the effects on the ABR of CNS pathology in the brainstem. ==Measurement techniques==

Recording parameters • Electrode montage: most performed with a vertical montage (high forehead [active or positive], earlobes or mastoids [reference right & left or negative], low forehead [ground] • Impedance: 5 kΩ or less (also equal between electrodes) • Filter settings: 30–1500 Hz bandwidth • Time window: 10ms (minimum) • Sampling rate: usually high sampling rate of circa 20 kHz • Intensity: usually start at 70 dBnHL • Stimulus type: click (100 us long), chirp or toneburst • Transducer type: insert, bone vibrator, sound field, headphones • Stimulation or repetition rate: 21.1 (for example) • Amplification: 100–150K • n (# of averages/ sweeps): 1000 minimum (1500 recommended) • Polarity: rarefaction or alternating recommended == Applications ==

The ABR is used for newborn hearing screening, auditory threshold estimation, intraoperative monitoring, diagnosing hearing loss type and degree, auditory nerve and brainstem lesion detection, and in development of cochlear implants. Site-of-lesion testing is sensitive to large acoustic tumors. ==Variants==

Stacked ABR Stacked ABR is the sum of the synchronous neural activity generated from five frequency regions across the cochlea in response to click stimulation and high-pass pink noise masking. In 2005, Don defined the Stacked ABR as "...an attempt to record the sum of the neural activity across the entire frequency region of the cochlea in response to a click stimuli." Small tumors may not sufficiently affect those fibers. However, MRI-ing every patient is not practical given its high cost, impact on patient comfort, and limited availability in many areas. In 1997, Don and colleagues introduced the Stacked ABR as a way to enhance sensitivity to smaller tumors. Their hypothesis was that the ABR-stacked derived-band ABR amplitude could detect tumors missed by standard ABRs. In 2005, Don stated that it would be clinically valuable to have available an ABR test to screen for small tumors. The Stacked ABR is sensitive, specific, widely available, comfortable, and cost-effective. Methodology The stacked ABR is a composite of activity from ALL frequency regions of the cochlea – not just high frequency. Tone-burst ABR Tone-burst ABR is used to obtain thresholds for children who are too young to otherwise reliably respond behaviorally to frequency-specific acoustic stimuli. The most common frequencies tested are 500, 1000, 2000, and 4000 Hz, as these frequencies are generally necessary for hearing aid programming. Auditory steady-state response Auditory steady-state response (ASSR) is an auditory evoked potential, elicited with modulated tones that can be used to predict hearing sensitivity in patients of all ages. It is an electrophysiologic response to rapid auditory stimuli and creates a statistically valid estimated audiogram (evoked potential predicts hearing thresholds). ASSR uses statistical measures to identify thresholds is a "cross-check" for verification purposes prior to arriving at a differential diagnosis. In 1981, Galambos and colleagues reported on the "40 Hz auditory potential" which is a continuous 400 Hz tone sinusoidally 'amplitude modulated' at 40 Hz and at 70 dB SPL. This produced a frequency-specific response, but the response was influenced by state of arousal. In 1991, Cohen and colleagues learned that by presenting at >70 Hz, the response was smaller, but less affected by sleep. In 1994, Rickards and colleagues showed that it was possible to obtain responses in newborns. In 1995, Lins and Picton found that simultaneous stimuli presented at rates in the 80 to 100 Hz range made it possible to obtain auditory thresholds. Comparison Similarities: • Both record bioelectric activity from electrodes arranged in similar arrays. • Both use auditory evoked potentials. • Both use acoustic stimuli delivered through inserts (preferably). • Both can be used to estimate thresholds for patients who cannot or will not participate in traditional behavioral measures. Differences: • ASSR looks at amplitude and phases in the spectral (frequency) domain rather than at amplitude and latency. • ASSR depends on peak detection across a spectrum rather than across a time vs. amplitude waveform. • ASSR is evoked using repeated sound stimuli presented at a high repetition rate rather than an abrupt sound at a relatively low rate. • ABR typically uses click or tone-burst stimuli in one ear at a time, but ASSR can be used binaurally while evaluating broad bands or four frequencies (500, 1k, 2k, & 4k) simultaneously. • ABR estimates thresholds basically from 1-4k in typical hearing losses. ASSR can estimate thresholds in the same range, but offers more frequency specific information more quickly and can estimate hearing in the severe-to-profound hearing loss ranges. • ABR depends upon a subjective analysis of the amplitude/latency function. ASSR uses a statistical analysis. • ABR is measured in microvolts (millionths of a volt) while ASSR is measured in nanovolts (billionths of a volt). ==Hearing aid fittings==

In certain cases where behavioral thresholds cannot be attained, ABR thresholds can be used for hearing aid fittings. Fitting formulas such as DSL v5.0 allow the hearing aid settings to be based on the ABR thresholds. Correction factors exist for converting ABR thresholds to behavioral thresholds, but vary greatly. For example, one set involves lowering ABR thresholds from 1000 to 4000 Hz by 10 dB and lowering the ABR threshold at 500 Hz by 15 to 20 dB. Previously, brainstem audiometry was used for hearing aid selection by using normal and pathological intensity-amplitude functions to determine appropriate amplification. The principal idea was based on the assumption that amplitudes of the brainstem potentials were directly related to loudness perception. Under this assumption, the amplitudes of brainstem potentials stimulated by the hearing devices should exhibit close-to-normal values. ABR thresholds do not necessarily improve in the aided condition. ABR can be an inaccurate indicator of hearing aid benefit due to difficulty processing the appropriate amount of fidelity of the transient stimuli used to evoke a response. Bone conduction ABR thresholds can be used if other limitations are present, but thresholds are not as accurate as ABR thresholds recorded through air conduction. Advantages: • evaluation of loudness perception in the dynamic range of hearing (recruitment) • determination of basic hearing aid properties (gain, compression factor, compression onset level) • cases with middle ear impairment (contrary to acoustic reflex methods) • non-cooperative subjects even in sleep • sedation or anesthesia without influence of age and vigilance (contrary to cortical evoked responses). Disadvantages: • in cases of severe hearing impairment including no or only poor information as to loudness perception • no control of compression setting • no frequency-specific hearing compensation ==Cochlear implantation and central auditory development==

Some 188,000 people around the world have cochlear implants. In the United States, 30,000 adults and over 30,000 children have them. In 1961, House began work on the predecessor of cochlear implants. House is an otologist. The first implant was approved by the FDA in 1984. It was a single-channel device and led to multi-channel cochlear implants. Cochlear implants transforms sound received by the implant's microphone into radio waves using the external sound processor. The external transmitting coil transmits the (frequency-modulated) radio waves through the skin. The signal is not turned back into sounds. The internal receiver stimulator delivers the correct electrical stimulation to the appropriate internal electrodes to represent the sounds. The electrode array stimulates auditory nerve fibers in the cochlea, which carry the signal to the brain. One way to measure the status of the auditory cortical pathways is to study the latency of cortical auditory evoked potentials (CAEP). In particular, the latency of the first positive peak (P1) of the CAEP is of interest. P1 is a robust positive wave occurring at around 100 to 300 ms in children. P1 latency represents the synaptic delays throughout the peripheral and central auditory pathways. P1 in children is considered a marker for maturation of the auditory cortical areas. P1 latency changes as a function of age, and is considered an index of cortical auditory maturation. P1 latency and age have a strong negative correlation, decrease in P1 latency with increasing age. This is most likely due to more efficient synaptic transmission over time. The P1 waveform also broadens with age. P1 neural generators are thought to originate from the thalamo-cortical portion of the auditory cortex. P1 may be the first recurrent activity in the auditory cortex. The negative component following P1 is called N1. N1 is not consistently seen in children until 12 years or age. A 2006 study measured the P1 response in deaf children who received cochlear implants at different ages to examine the limits of plasticity in the central auditory system. and 2007 ==See also==